This disclosure generally relates generally relates to rechargeable batteries, and more specifically, to systems and methods for health management of rechargeable batteries for aerospace applications, automotive applications, and other suitable applications.
There are significant cost and maintenance challenges associated with rechargeable batteries. These challenges may impose economic concerns in a wide variety of applications. For example, in aerospace applications, unexpected battery failures may present a considerable economic impact due to system interruptions, unscheduled flight delays and cancellations, loss of capabilities, and associated costs and logistical challenges.
The general problem is to provide safe, cost-effective storage of electrical energy. In many modern aerospace products, this storage function can be best achieved using chemical batteries, such as lithium ion batteries. These batteries pose a variety of technical challenges. One challenge is monitoring the health of the battery so it can be serviced or replaced before it becomes unreliable. Ideally, it would be possible to monitor the health of a battery in situ, i.e., without removing it from its normal position such as in an airplane equipment rack. A second choice—less desirable, but acceptable—is to remove the battery from its mounting position, but not disassemble it or remove it from the airplane.
A major factor affecting the health of a battery is the amount of liquid electrolyte contained in each cell. Typical cells, such as modern lithium-ion cells, are partially filled with a liquid electrolyte. It is known that such cells can lose some of their required electrolyte, for example, due to leakage through tiny gaps in terminal seals or vent plugs. Electrolyte is a mixture of chemicals. When electrolyte leaks out, it sometimes does so as a vapor passing through a tiny passage. Smaller molecules with high vapor pressure leak out faster, leaving behind larger molecules with low vapor pressure. This changes the composition of the electrolyte, which affects its viscosity (and therefore its ionic conductivity), its heat transfer properties, its dielectric constant, and its dielectric breakdown strength. These changes all degrade the health of the battery.
It is also possible that an electrochemical cell will have extra liquid, for example, due to absorption of water from the atmosphere through tiny gaps. Moisture in the cell can lead to chemical reactions, e.g., in a lithium-ion battery, these reactions can form hydrofluoric acid that corrodes surfaces within the cell and causes it to fail. Some battery chemistries use hygroscopic electrolytes with very low vapor pressure. Leaky cells with these chemistries tend to gain mass as they absorb water from the air.
Not all changes in electrolyte level are due to leaks, and not all changes in level are bad. For example, the electrolyte level may change as the electrode degrades due to aging or cycling, so that it accommodates less electrolyte within its porous surface. This is an indication of degraded cell health, but not a leak. A typical electrode also absorbs and releases electrolyte as the cell charges or discharges. This is normal; indeed, if the electrolyte level fails to rise or fall with the state of charge, that would be an indication of degraded health. Therefore, one cannot conclude that high or low electrolyte levels indicate poor health without knowing the battery's state of charge.
A further complication is that an electrochemical cell is typically not used by itself. Cells are connected in series and/or parallel to form a battery. Opening a battery is a costly maintenance activity, so methods for monitoring battery health without opening up the battery are preferred. Assessing the health of individual cells without opening the battery is difficult. Or example, one or more of the cells may be missing liquid. The overall mass of the battery may be unchanged, however, because moisture has condensed inside the battery, or because the leaking electrolyte is trapped within the battery. Even if a maintainer can tell that at least one cell is missing liquid, the maintainer cannot tell which cell or cells has lost liquid until he or she removes each cell from the battery and tests it.
The following methods for monitoring battery health are known:
1. Weigh the battery. This approach can tell if one or more cells in the battery have lost electrolyte which has also escaped from the battery, assuming that no moisture has condensed in the battery and offset the weight loss. This method cannot determine which cells might have lost electrolyte.
2. Weigh the cells. This approach can tell if an individual cell has lost or gained liquid. This method requires opening the battery, removing every cell, and weighing each cell. This costs skilled labor time. This method cannot determine whether the electrode is capable of absorbing and releasing a healthy amount of liquid during each charge-discharge cycle.
3. Electrochemical impedance spectroscopy of the battery. This approach can reveal some aspects of battery health. If a cell has gone bad, electrochemical impedance spectroscopy (EIS) cannot tell which cell or cells in the battery have done so. The EIS equipment is typically expensive.
4. Electrochemical impedance spectroscopy of the cells. This approach can reveal some aspects of cell health. This technique requires opening the battery, removing every cell, and testing each cell. This costs skilled labor time.
5. X-ray backscatter. This approach can reveal liquid levels in each individual cell. Because x-ray backscatter equipment is large and cumbersome, this approach requires removing the battery from the vehicle and transporting it to a lab. This costs skilled labor time. The equipment has a high capital cost and poses radiation safety risks.
There is room for improving upon the above-described known methods of battery health monitoring.